The invention relates to a method for proximity effect correction, e.g. as may be performed in a charged particle lithography system. Typically, in a charged particle beam lithography system at least one beam of charged particles is directed onto a resist-layer of a wafer to form a desired pattern in the resist. The achievable pattern resolution within the resist depends on how well the spatial charged particle energy deposition can be controlled within the resist. When a charged particle beam is directed onto a position on a substrate which is coated with resist, some of the incident charged particles are scattered on their trajectories through the resist.
In forward scattering, a charged particle of the beam of charged particles may collide with an electron of the substrate or the resist. This causes the charged particle to be deflected from its trajectory and to deposit part of its energy in the substrate or resist.
The charged particles may also collide with a nucleus of an atom in the substrate or resist, resulting in a substantially elastic backscattering event which causes the charged particle to be deflected to a much greater extent than it would be when colliding with an electron.
Due to forward scattering and backscattering of the charged particles, the actual dose, or energy deposition, and thus the developed pattern, is wider than the desired pattern scanned by the charged particle beam on the surface of the resist. This phenomenon is called the proximity effect. When modelling the proximity effect typically use is made of a point spread function, which is often referred to as a proximity effect function. The point spread function depends on factors such as the materials of the target and the resist used, resist thickness, the primary beam energy and/or the development process used for developing the resist. When these factors are known, a corresponding point spread function may be calculated without empirically determining the point spread function. Alternatively the point spread function may be estimated using empirical methods, an brief overview of which is given in the article “Experimental study of proximity effect corrections in electron beam lithography”, Jianguo Zhu et al., Proc. SPIE vol. 2437, Electron-Beam, X-Ray, EUV, and Ion-Beam Submicrometer Lithographies for Manufacturing V, pg. 375 (May 19, 1995); doi: 10.1117/12.209175.
U.S. Pat. No. 7,638,247 B2, which is incorporated herein by reference, describes a method for performing an electron beam proximity correction process in which both a short range (caused by forward scattering) and a long range (caused by backscattering) proximity effect correction is carried out on a received layout, wherein a dose value is formulated for each feature using the results of both the short range and the long range proximity effect corrections. In an embodiment, the long range proximity effect correction is carried out using a grid-based deconvolution method.
Applicant has found that when the received layout contains high spatial frequencies, e.g. contains very small features, known methods which make use of deconvolution or approximations thereof are susceptible to errors which lead to undesired variations in the calculated value of the dose which is to be delivered to the resist. It is an object of the present invention to provide an improved method for charged particle beam proximity effect correction.